12 



ELEMENTARY STUDIES IN BOTANY 



tions by Pleurococcus (Fig. 2), very common as green 

 patches on damp tree trunks, old board fences, damp walls 

 and rocks, etc. When material from these patches, which 

 often look like green stains, is observed under the mi- 

 croscope, it is discovered to be made up of innumerable 



green, spherical cells. The figure 

 referred to (Fig. 2) shows a sin- 

 gle individual and also the suc- 

 cessive divisions that result in 

 groups of individuals. In every 

 case, each cell is a separate plant, 

 quite independent of all the rest. 

 In each plant (cell) shown in the 

 figure the nucleus may be seen, 

 surrounded by the granular-look- 

 ing cytoplasm, and this in turn 

 invested by a wall. It is evident 

 that these minute individuals are 

 equipped to do the work of nu- 

 trition and of reproduction just 

 as truly as are the larger plants. 

 A second stage is represented 

 by those Algae whose bodies are 

 also single cells, but the cells cling 



FIG. 2. Pleurococcus; in the up- 

 per left-hand corner is a single 

 plant (one cell), with its nu- 

 cleus and cytoplasm surrounded 

 by a cell-wall ; in other figures 

 the cell has divided, and has 

 given rise to loose, irregular 

 groups, in which each cell is an . 



independent individual, and together in such definite groups 

 Jhe^eslr " 168 separated from that the groups are called colo- 

 nies. Examples of such plants 



are shown in Figs. 3 and 4, In Fig. 3 (Gkeotfyece) , 

 the cells, as they are formed, are held together in a more 

 or less irregular colony by a mucilage that is developed 

 from the cell- wall material (cellulose). In Fig. 4, two- 

 colonies are shown: Nostoc (A), with the cells (individ- 

 uals) held together in a definite row, so that the colony 

 resembles a string of beads, and the mucilage is so abundant 

 that many such colonies may be imbedded together in a single 



